I. Field of the Invention
This invention relates generally to pulleys and more particularly, to a timing pulley for accommodating multiple timing belts thereon.
II. Discussion of the Prior Art
Pulleys change the direction and point of application of a pulling force and may be used for transmitting rotation from one shaft, the driving shaft, to a driven shaft. In an ordinary pulley and belt configuration, the belt is wrapped around a driving pulley and a driven pulley. The driving pulley may be connected, for example, to a motor's output shaft and the driven pulley may be connected to an auxiliary device. The two pulleys and belt transfer rotation from the motor's output shaft to the auxiliary device's shaft. In this situation, the pulleys turn at a particular rate in relation to their relative diameters and timing information may be passed between the shafts, as long as slippage does not occur.
A conventional pulley has a single groove in its outer rim or periphery for accepting a rope or belt. The belt is usually smooth and shaped to fit the smooth groove of the pulley. A friction fit, between the pulley and the belt, is accomplished by adjusting the distance between the driving and the driven pulleys to pull the belt taut. This friction fit should be sufficient to prevent the belt from slipping but not so taut as to place undue stress on the bearings of the shafts on which the pulleys are mounted. Unfortunately, a smooth groove and belt configuration loosens over time and slippage often occurs. This slippage leads to a loss of drive power and timing information. Where timing information is critical, slippage cannot be tolerated.
The contact area between the belt and pulley may be increased to reduce the tendency toward slippage. A wider grooved rim has a wider flat surface on its inside diameter back wall for contacting a wider belt. A deeper grooved rim has deeper side surfaces for contacting the sides of the belt. Another way of providing more surface area to reduce slippage is to have multiple pulleys and belts offset laterally relative to one another on the driving and the driven shafts. In this situation, the total contacting surface area is the sum of the surface areas of the individual pulley and belt combinations. The possibility of slippage is greatly reduced by using multiple offset pulleys and belts. However, none of these configurations insure against slippage and the loss of timing information.
Multiple pulleys and belts may be needed for driving multiple auxiliary devices. However, laterally arranged pulleys take extra space. In applications where space is limited, it is more convenient to have a number of pulleys and belts in a common plane.
A device using multiple pulleys in a common plane is shown and described in U.S. Pat. No. 2,548,316, issued to Locke. The '316 patent discloses a drive pulley mounted on a drive shaft and driving an outer belt and an inner belt. The inner belt is wrapped around the drive pulley and an idler pulley. The outer belt is wrapped around the drive pulley, outside the inner belt, and also around an auxiliary device pulley. The pulleys lie in a common plane, with the idler pulley between the drive pulley and the auxiliary device pulley. The outer and inner belts are V shaped and fit into a V groove in the drive pulley. The outer belt lies on the inner belt and gets traction from its engagement with the inner belt for driving the auxiliary device pulley. Thus, the traction for the outer belt is enhanced by the inner belt. This traction, however, is not absolute and slippage may occur as the outer belt loosens over time.
A variation on overlapping belts is shown in U.S. Pat. No. 4,634,403, issued to Peabody et al. The '403 patent discloses a smooth V shaped belt fitting into an inner groove and a smooth flat outer belt riding over the inner belt and fitting into an outer groove. In this configuration, the inner belt is in contact with the sides of the V groove. The outer belt holds the inner belt in place and receives traction from the top surface of the inner belt and the flat surfaces of the outer groove. The outer belt, however, is subject to stretching and slippage is possible.
Other overlapping belt systems are shown in U.S. Pat. Nos. 3,965,764, and 3,981,205. In the '764 patent an inner belt rides in the V groove of an ordinary nonadjustable pulley and drives one auxiliary device. The outer belt rides on top of the inner belt and drives two auxiliary pulleys. All of the pulleys lie in the same plane. In the '205 patent the inner belt rides in the V groove of an adjustable pulley and the outer belt rides on top of the inner belt without touching the sides of the adjustable pulley. As the adjustable pulley expands to decrease its diameter, the inner belt remains in contact with the sides of the adjustable pulley and the outer belt continues to ride on top of the inner belt. The inner and outer belts drive various auxiliary devices. In both of these systems, either belt may slip and lose timing information.
Traction is enhanced by having an inner belt in contact with an outer belt. However, in each of the prior art arrangements described, the inner belt is basically smooth and the outer belt has either smooth surfaces or grooves that run the length of the belt. Although traction is enhanced by having an inner belt in contact with an outer belt, the smooth surfaces of the belt and pulley may slip. In situations where timing information is critical slippage cannot be tolerated.
To avoid slippage, so-called timing pulleys and timing belts having meshing teeth or protuberances are well known in the art. For example, timing belts or chains have been used on automobiles for years for synchronizing the movement of engine components. The teeth prevent slippage and the loss of timing information. However, timing belts are not typically designed for withstanding a large amount of torque. Thus, driving multiple auxiliary devices with a single belt may cause the belt's teeth to rip apart. Laterally offset timing pulleys may be used to drive multiple devices. But, where space is a limitation and laterally offset pulleys cannot be used, a timing pulley capable of accommodating multiple belts in a common plane is needed.
It is accordingly a principle object of this invention to provide a pulley for accommodating multiple timing belts.
Another object of the invention is to provide a pulley for accommodating multiple timing belts in a common plane.
Yet another object of the invention is to provide a timing pulley having a stepped belt engaging surface defining inner and outer diameters in a grooved rim for accommodating multiple timing belts.
Yet another object of the invention is to provide a timing pulley having at least one inner belt engaging surface recessed with respect to an outer belt engaging surface so the inner timing belt runs inside and is overlapped by the outer timing belt.